The present invention relates to an optical layer stack as it may, for example, be used for optoelectronic systems and, in particular, to the mechanical spacing of individual layers of the optical layer stacks.
Optoelectronic systems or layer stacks as they are schematically illustrated in FIGS. 10a to 10c are, for example used in digital cameras, wireless devices having a photography function and many other applications. In the manufacturing of opto-electronic systems, e.g., micro objectives for mobile telephones, tight manufacturing and adjustment tolerances in a range of a few micrometers (μm) have to be adhered to. In the manufacturing of such optoelectronic systems in panels, i.e., on wafer level, this means that individual layers or sheets of the optoelectronic system (e.g., wafers having lenses 1002, spacer wafers 1004 for realizing air spaces or optically used areas 1006) are to be manufactured with a high mechanical precision as manufacturing tolerances have an influence on the optical characteristics of the optoelectronic system. This among others leads to high manufacturing costs or little process yield.
Needed layers or sheets of lens and so-called spacer wafers are manufactured individually according to many different methods. Lenses 1002 advantageously consist of UV-curable polymer and are arranged on a glass substrate. Several of these glass substrates 1008 are then stacked onto each other and advantageously joined by means of UV-curable adhesive. Needed air spaces 1006 between the lenses 1002 are generated by the spacer wafers 1004 representing spacer layers comprising through holes.
Tight axial position tolerances of the lens areas resulting from a function of the optical layer stack here have to be met by mechanical thickness tolerances of the individual layers and the thickness tolerances of the adhesive layers. Thus, high requirements to dimensional accuracy of mechanical components result which contribute to setting the spacing of optical components 1002, e.g., lenses, only restrictedly, with respect to the optical function of the optical or optomechanical layer stack. Here, advantageously glass materials are used as spacer layers 1004 (spacer wafers) as the same fulfill a requirement with respect to high temperature resistance. In a monolithic implementation, i.e., no use of glass wafers, temperature resistant polymers are used. In both cases structures used for spacing the optically effective areas are transparent, which may be a disadvantage for the optical function as a result of the penetration of false light.
The different materials for layers having optically effective elements, spacer layers and joining layers are disadvantageous with respect to climate and long-term performance of the resulting optoelectronic system or the optical or optomechanical layer stack used therein. For example, polymers for the manufacturing of optical or micro-optical components by UV replication have a high thermo-optical coefficient, i.e., the refractive index of the material strongly changes with a changing temperature, wherein generally with an increasing temperature the refractive index decreases. The thermo-optical coefficients of such materials are approximately 10 to 100 times higher than those of glass materials which are otherwise used in optics. Consequently, the refractive power of a lens of UV polymer materials may substantially reduce with temperature changes which, in case of imaging optics, leads to an increase of the image-side focal length. When maintaining the spacing between lens and image, a defocusing and thus a deterioration of imaging quality results.
As a consequence of the increase of the focal length with an increasing temperature caused by a thermal expansion of the lens material and the dependence of the refractive index on the temperature, an increase of the focal length between the last lens and the image position results. As a consequence of thermal expansion of the spacer layers, this spacing is also increased, in principle a compensation of thermal defocusing (athermization) may be achieved. For lens materials with a small thermo-optical coefficient, e.g., glass, this is possible using spacer materials having adapted thermal expansion coefficients. In contrast to this, for athermization of objectives having plastics lenses, materials having a coefficient of expansion of some 100×10−6/K are needed which are not known in conventional technology.